When a user registered a notification mail sent to that users mail id when it is set to admin approval is required. If you want to send a notification mail to the admin mail or a custom mail do the following,
Go to /admin/config/system/actions. In the section "Create an advanced action" choose "send an email" and click on "create". It takes you to a page where you need to enter a recipients email address (Enter the Admin email or your custom email address here), subject and Message, then click save.
Now go to /admin/structure/trigger/user (if you not enabled Trigger module please enable it). In the Section "Trigger: After creating a new user account" Choose "Send e-mail" in the select box. Click Assign.
Go to /admin/config/system/actions. In the section "Create an advanced action" choose "send an email" and click on "create". It takes you to a page where you need to enter a recipients email address (Enter the Admin email or your custom email address here), subject and Message, then click save.
Now go to /admin/structure/trigger/user (if you not enabled Trigger module please enable it). In the Section "Trigger: After creating a new user account" Choose "Send e-mail" in the select box. Click Assign.
